Tips for the testing of Dissolved Oxygen:
Dissolved Oxygen
Oxygen is critical to the survival of aquatic plants and animals, and a shortage of dissolved oxygen is not only a sign of pollution, it is harmful to the fish. Some aquatic species are more sensitive to oxygen depletion than others, but some general guidelines to consider when analyzing test results are:
5-6 ppm Sufficient for most species
<3 ppm Stressful to most aquatic species
<2 ppm Fatal to most species
Because of its importance to the fish’s survival, aquaculturists, or “fish farmers,” and aquarists use the dissolved oxygen test as a primary indicator of their system’s ability to support healthy fish.
Where Does The Oxygen Come From?
The oxygen found in water comes from many sources, but the largest source is oxygen absorbed from the atmosphere. Wave action and splashing allows more oxygen to be absorbed into the water. A second major source of oxygen is aquatic plants, including algae; during photosynthesis plants remove carbon dioxide from the water and replace it with oxygen.
Absorption:
Oxygen is continuously moving between the water and surrounding air. The direction and speed of this movement is dependent upon the amount of contact between the air and water. A tumbling mountain stream or windswept, wave covered lake, where more of the water’s surface is exposed to the air, will absorb more oxygen from the atmosphere than a calm, smooth body of water. This is the idea behind aerators; by creating bubbles and waves the surface area is increased and more oxygen can enter the water.
Photosynthesis:
In the leaves of plants one of the most important chemical processes on Earth is constantly occurring- photosynthesis. During daylight, plants constantly take carbon dioxide from the air, and, in the presence of water, convert it to oxygen and carbohydrates, which are used to produce additional plant material. Since photosynthesis requires light, plants do not photosynthesize at night, so no oxygen is produced. Chemically, the photosynthesis reaction can be written as:
Light + nCO2 + nH2O
(CH2O)n + nO2
Light + Carbon Dioxide + Water
Carbohydrate + Oxygen
Where Does The Oxygen Go?
Once in the water, oxygen is used by the aquatic life. Fish and other aquatic animals need oxygen to breathe or respire. Oxygen is also consumed by bacteria to decay, or decompose, dead plants and animals.
Respiration:
All animals, whether on land or underwater, need oxygen to respire, and grow and survive. Plants and animals respire throughout the night and day, consuming oxygen and producing carbon dioxide, which is then used by plants during photosynthesis.
Decomposition:
All plant and animal waste eventually decomposes, whether it is from living animals or dead plants and animals. In the decomposition process, bacteria use oxygen to oxidize, or chemically alter, the material to break it down to its component parts. Some aquatic systems may undergo extreme amounts of oxidation, leaving no oxygen for the living organisms, which eventually leave or suffocate.
Other Factors:
The oxygen level of a water system is not only dependent on production and consumption. Many other factors work together to determine the potential oxygen level, including:
- Salty vs. fresh water: Fresh water can hold more oxygen than salt water.
- Temperature: Cold water can hold more oxygen than warm water.
- Atmospheric pressure (Altitude): The greater the atmospheric pressure the more oxygen the water will hold.
